I totally agree about USED bookstores, they tend to actually care about the business. They also tend to have great prices, often equal to Amazon's when you count shipping.
Sadly, in Barnes and Noble, all you have are cashiers, who are there to take your money and otherwise don't give a darn about anything. Since you get absolutely NOTHING extra from them, I feel no need to to spend a cent there if I get a better deal elsewhere.
When it comes to a business, like cars and cameras and other stuff where you spend a lot of time discussing what's best suited to your needs, unless you can find a ridiculously better deal elsewhere, I would buy there. I'm not going to hunt all over creation to see how I can save a few $. In fact, I might go so far as to say, I feel they should get a little something extra. For example, when you go to a restaurant, you tip the waitress, depending of course on how well they serve you. Because that service is worth something to you. If the car salesman helped you make a better, more informed choice, he deserves the money, WITHIN REASON. I'm not saying you have to let yourself be ripped off.
I just think in any such situation, you should be honest with yourself, ask how much you have been helped, what's a reasonable thank you to offer, and then be willing to pay maybe a little extra FOR THE SERVICE YOU'VE BEEN GIVEN.
Certainly, if you decide you don't really want anything after all, I'm not suggesting you must.
In that little mom & pop used bookshop where I can discuss books with the cashier, and get steered maybe toward a new author I will enjoy, then there too I would spend, maybe 50¢ more on a $3 book? It's like a tip. In Barnes & Noble they aren't doing anything to deserve anything, any more than a grocery store cashier.
The good news is, of course, that in a used bookstore, I'm likely to find the best prices anyway.
